This invention relates to apparatus and methods for testing a telephone circuit protector, such as a five-pin central office protector.
A telephone central office's primary line of defense from outside high voltages (e.g., lightning) includes voltage arresters found in central office protectors. Such protectors can also include current shunts to protect against hazardous currents. These protectors are typically located on a frame interface to an outside plant transmission facility.
A principal function of a protector is to safeguard operating personnel and connected equipment from hazardous longitudinal potentials and currents. The hazardous voltages are discharged to ground via the voltage arresters, and the hazardous currents are grounded out after heating or otherwise closing the shunts to ground.
One type of protector includes air-gapped carbon arresters. Such an arrester includes two ceramic mounted blocks of carbon separated by an air gap. The gap distance is directly proportional to the voltage necessary to arc across (breakdown) the gap. Carbon arresters with nominal 3-mil gaps (450 volt) are characterized by large statistical variations in breakdown voltages and life expectancies in the 100's. Representative .+-.3.sigma. breakdown voltage ranges are 300 to 700 volts on surges with 2000 V/sec (DC breakdown) and 360 to 950 volts on surges with 100 V/usec (fast impulse breakdown) rate-of-rise. Two standard breakdown voltages are 350 V and 450 V.
Another type of protector includes gas tube arresters. Unlike air-gapped carbon arresters, the performance characteristics of gas tube arresters can be relatively tightly controlled. This is due to the gas tube's sealed environment, metal/ceramic construction, and wider gap filled with ionizing gas. Gas tubes with a nominal DC breakdown voltage rating of 450 volts typically have a smaller statistical variation in their breakdown voltages. Representative .+-.3.sigma. breakdown voltage ranges are 400 to 500 volts.
Another type of protector, which is gaining in popularity, is the solid-state protector. This type typically has even smaller or tighter tolerances regarding variation in breakdown voltage.
In addition to having voltage arresters, each of these types of protectors can include a circuit through which potentially damaging currents, at least some of which are referred to as sneak currents, can be shunted to ground. One type of current limiting circuit includes a heat coil which heats an associated temperature-responsive shunt element. When a sufficient temperature is reached, the shunt closes to conduct the current to ground.
For these protectors to protect, their voltage and current limiting circuits must function properly. Thus, there is the need for an apparatus and method for non-destructively testing the protectors. Although there are devices which test at least some aspects of these types of protectors, we are not aware of a test apparatus or method which tests a voltage limiting circuit and a current limiting circuit for the breakdown voltage of the voltage limiting circuit, for continuity of the current limiting circuit, and for ground faults, during a single test period and which also visually displays the breakdown voltage and continuity and ground fault indications. There is a need for such an apparatus and method so that these types of protectors can be more completely and efficiently tested.